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Employee Benefit Plans - Changes In Benefit Obligations Recognized In Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Pension Plans      
Pension Plans Postretirement And Other Employee Benefits [Line Items]      
Net actuarial gain (loss) $ (38.0) $ 2.7 $ (103.9)
Settlements     (0.3)
Net actuarial losses 11.8 14.1 10.3
Prior service cost 0.2 0.4 0.5
Net transition obligations     0.1
Total (26.0) 17.2 (93.3)
OPEB      
Pension Plans Postretirement And Other Employee Benefits [Line Items]      
Net actuarial gain (loss) 0.1 0.2 (0.8)
Net actuarial losses 0.2 0.3 0.2
Prior service cost (0.8) (0.8) (0.9)
Total $ (0.5) $ (0.3) $ (1.5)